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  <h1>Source code for torch.multiprocessing.spawn</h1><div class="highlight"><pre>
<span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">absolute_import</span><span class="p">,</span> <span class="n">division</span><span class="p">,</span> <span class="n">print_function</span><span class="p">,</span> <span class="n">unicode_literals</span>

<span class="kn">import</span> <span class="nn">multiprocessing</span>
<span class="kn">import</span> <span class="nn">multiprocessing.connection</span>
<span class="kn">import</span> <span class="nn">signal</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">import</span> <span class="nn">warnings</span>

<span class="kn">from</span> <span class="nn">.</span> <span class="kn">import</span> <span class="n">_prctl_pr_set_pdeathsig</span>


<span class="k">def</span> <span class="nf">_wrap</span><span class="p">(</span><span class="n">fn</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">error_queue</span><span class="p">):</span>
    <span class="c1"># prctl(2) is a Linux specific system call.</span>
    <span class="c1"># On other systems the following function call has no effect.</span>
    <span class="c1"># This is set to ensure that non-daemonic child processes can</span>
    <span class="c1"># terminate if their parent terminates before they do.</span>
    <span class="n">_prctl_pr_set_pdeathsig</span><span class="p">(</span><span class="n">signal</span><span class="o">.</span><span class="n">SIGINT</span><span class="p">)</span>

    <span class="k">try</span><span class="p">:</span>
        <span class="n">fn</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">)</span>
    <span class="k">except</span> <span class="ne">KeyboardInterrupt</span><span class="p">:</span>
        <span class="k">pass</span>  <span class="c1"># SIGINT; Killed by parent, do nothing</span>
    <span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>
        <span class="c1"># Propagate exception to parent process, keeping original traceback</span>
        <span class="kn">import</span> <span class="nn">traceback</span>
        <span class="n">error_queue</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">traceback</span><span class="o">.</span><span class="n">format_exc</span><span class="p">())</span>
        <span class="n">sys</span><span class="o">.</span><span class="n">exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>


<span class="c1"># Multiprocessing contexts are introduced at Python 3.4</span>
<span class="n">_supports_context</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">version_info</span> <span class="o">&gt;=</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>


<span class="k">def</span> <span class="nf">_python_version_check</span><span class="p">():</span>
    <span class="k">if</span> <span class="ow">not</span> <span class="n">_supports_context</span><span class="p">:</span>
        <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">&quot;Requires python 3.4 or higher to use &quot;</span>
                           <span class="s2">&quot;torch.multiprocessing.spawn and &quot;</span>
                           <span class="s2">&quot;torch.multiprocessing.ProcessContext helper &quot;</span>
                           <span class="s2">&quot;to launch multiple processes. If you are using &quot;</span>
                           <span class="s2">&quot;this for distributed training and have a lower &quot;</span>
                           <span class="s2">&quot;version of python, please use &quot;</span>
                           <span class="s2">&quot;torch.distributed.launch instead.&quot;</span><span class="p">)</span>


<span class="k">class</span> <span class="nc">ProcessContext</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">processes</span><span class="p">,</span> <span class="n">error_queues</span><span class="p">):</span>
        <span class="n">_python_version_check</span><span class="p">()</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">error_queues</span> <span class="o">=</span> <span class="n">error_queues</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">processes</span> <span class="o">=</span> <span class="n">processes</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">sentinels</span> <span class="o">=</span> <span class="p">{</span>
            <span class="n">process</span><span class="o">.</span><span class="n">sentinel</span><span class="p">:</span> <span class="n">index</span>
            <span class="k">for</span> <span class="n">index</span><span class="p">,</span> <span class="n">process</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">processes</span><span class="p">)</span>
        <span class="p">}</span>

    <span class="k">def</span> <span class="nf">pids</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="p">[</span><span class="nb">int</span><span class="p">(</span><span class="n">process</span><span class="o">.</span><span class="n">pid</span><span class="p">)</span> <span class="k">for</span> <span class="n">process</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">processes</span><span class="p">]</span>

    <span class="k">def</span> <span class="nf">join</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">timeout</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
        <span class="sa">r</span><span class="sd">&quot;&quot;&quot;</span>
<span class="sd">        Tries to join one or more processes in this spawn context.</span>
<span class="sd">        If one of them exited with a non-zero exit status, this function</span>
<span class="sd">        kills the remaining processes and raises an exception with the cause</span>
<span class="sd">        of the first process exiting.</span>

<span class="sd">        Returns ``True`` if all processes have been joined successfully,</span>
<span class="sd">        ``False`` if there are more processes that need to be joined.</span>

<span class="sd">        Arguments:</span>
<span class="sd">            timeout (float): Wait this long before giving up on waiting.</span>
<span class="sd">        &quot;&quot;&quot;</span>
        <span class="c1"># Ensure this function can be called even when we&#39;re done.</span>
        <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">sentinels</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
            <span class="k">return</span> <span class="kc">True</span>

        <span class="c1"># Wait for any process to fail or all of them to succeed.</span>
        <span class="n">ready</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">wait</span><span class="p">(</span>
            <span class="bp">self</span><span class="o">.</span><span class="n">sentinels</span><span class="o">.</span><span class="n">keys</span><span class="p">(),</span>
            <span class="n">timeout</span><span class="o">=</span><span class="n">timeout</span><span class="p">,</span>
        <span class="p">)</span>

        <span class="n">error_index</span> <span class="o">=</span> <span class="kc">None</span>
        <span class="k">for</span> <span class="n">sentinel</span> <span class="ow">in</span> <span class="n">ready</span><span class="p">:</span>
            <span class="n">index</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">sentinels</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="n">sentinel</span><span class="p">)</span>
            <span class="n">process</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">processes</span><span class="p">[</span><span class="n">index</span><span class="p">]</span>
            <span class="n">process</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
            <span class="k">if</span> <span class="n">process</span><span class="o">.</span><span class="n">exitcode</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">:</span>
                <span class="n">error_index</span> <span class="o">=</span> <span class="n">index</span>
                <span class="k">break</span>

        <span class="c1"># Return if there was no error.</span>
        <span class="k">if</span> <span class="n">error_index</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="c1"># Return whether or not all processes have been joined.</span>
            <span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">sentinels</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span>

        <span class="c1"># Assume failure. Terminate processes that are still alive.</span>
        <span class="k">for</span> <span class="n">process</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">processes</span><span class="p">:</span>
            <span class="k">if</span> <span class="n">process</span><span class="o">.</span><span class="n">is_alive</span><span class="p">():</span>
                <span class="n">process</span><span class="o">.</span><span class="n">terminate</span><span class="p">()</span>
            <span class="n">process</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

        <span class="c1"># There won&#39;t be an error on the queue if the process crashed.</span>
        <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">error_queues</span><span class="p">[</span><span class="n">error_index</span><span class="p">]</span><span class="o">.</span><span class="n">empty</span><span class="p">():</span>
            <span class="n">exitcode</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">processes</span><span class="p">[</span><span class="n">error_index</span><span class="p">]</span><span class="o">.</span><span class="n">exitcode</span>
            <span class="k">if</span> <span class="n">exitcode</span> <span class="o">&lt;</span> <span class="mi">0</span><span class="p">:</span>
                <span class="n">name</span> <span class="o">=</span> <span class="n">signal</span><span class="o">.</span><span class="n">Signals</span><span class="p">(</span><span class="o">-</span><span class="n">exitcode</span><span class="p">)</span><span class="o">.</span><span class="n">name</span>
                <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span>
                    <span class="s2">&quot;process </span><span class="si">%d</span><span class="s2"> terminated with signal </span><span class="si">%s</span><span class="s2">&quot;</span> <span class="o">%</span>
                    <span class="p">(</span><span class="n">error_index</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
                <span class="p">)</span>
            <span class="k">else</span><span class="p">:</span>
                <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span>
                    <span class="s2">&quot;process </span><span class="si">%d</span><span class="s2"> terminated with exit code </span><span class="si">%d</span><span class="s2">&quot;</span> <span class="o">%</span>
                    <span class="p">(</span><span class="n">error_index</span><span class="p">,</span> <span class="n">exitcode</span><span class="p">)</span>
                <span class="p">)</span>

        <span class="n">original_trace</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">error_queues</span><span class="p">[</span><span class="n">error_index</span><span class="p">]</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>
        <span class="n">msg</span> <span class="o">=</span> <span class="s2">&quot;</span><span class="se">\n\n</span><span class="s2">-- Process </span><span class="si">%d</span><span class="s2"> terminated with the following error:</span><span class="se">\n</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">error_index</span>
        <span class="n">msg</span> <span class="o">+=</span> <span class="n">original_trace</span>
        <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="n">msg</span><span class="p">)</span>


<div class="viewcode-block" id="SpawnContext"><a class="viewcode-back" href="../../../multiprocessing.html#torch.multiprocessing.SpawnContext">[docs]</a><span class="k">class</span> <span class="nc">SpawnContext</span><span class="p">(</span><span class="n">ProcessContext</span><span class="p">):</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">processes</span><span class="p">,</span> <span class="n">error_queues</span><span class="p">):</span>
        <span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s1">&#39;SpawnContext is renamed to ProcessContext since 1.4 release.&#39;</span><span class="p">)</span>
        <span class="nb">super</span><span class="p">(</span><span class="n">SpawnContext</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">processes</span><span class="p">,</span> <span class="n">error_queues</span><span class="p">)</span>
    <span class="k">pass</span></div>


<span class="c1"># Note: [start_processes]</span>
<span class="c1"># mp.start_processes handles both start_method=&#39;spawn&#39; and &#39;fork&#39;. It&#39;s supposed to be a</span>
<span class="c1"># more generalized API than mp.spawn. Currently we only document mp.spawn as it&#39;s the</span>
<span class="c1"># CUDA compatible start_method. However, in environments like Ipython notebooks, &#39;fork&#39;</span>
<span class="c1"># works better than &#39;spawn&#39;. Every helper function we created for mp.spawn is indeed</span>
<span class="c1"># general enough, and backends like XLA can reuse them in Colab notebooks as well.</span>
<span class="c1"># Currently we only add this API first, we can consider adding it to documentation as</span>
<span class="c1"># needed in the future.</span>
<span class="k">def</span> <span class="nf">start_processes</span><span class="p">(</span><span class="n">fn</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">nprocs</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">join</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">daemon</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">start_method</span><span class="o">=</span><span class="s1">&#39;spawn&#39;</span><span class="p">):</span>
    <span class="n">_python_version_check</span><span class="p">()</span>
    <span class="n">mp</span> <span class="o">=</span> <span class="n">multiprocessing</span><span class="o">.</span><span class="n">get_context</span><span class="p">(</span><span class="n">start_method</span><span class="p">)</span>
    <span class="n">error_queues</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="n">processes</span> <span class="o">=</span> <span class="p">[]</span>
    <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">nprocs</span><span class="p">):</span>
        <span class="n">error_queue</span> <span class="o">=</span> <span class="n">mp</span><span class="o">.</span><span class="n">SimpleQueue</span><span class="p">()</span>
        <span class="n">process</span> <span class="o">=</span> <span class="n">mp</span><span class="o">.</span><span class="n">Process</span><span class="p">(</span>
            <span class="n">target</span><span class="o">=</span><span class="n">_wrap</span><span class="p">,</span>
            <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="n">fn</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">error_queue</span><span class="p">),</span>
            <span class="n">daemon</span><span class="o">=</span><span class="n">daemon</span><span class="p">,</span>
        <span class="p">)</span>
        <span class="n">process</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
        <span class="n">error_queues</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">error_queue</span><span class="p">)</span>
        <span class="n">processes</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">process</span><span class="p">)</span>

    <span class="n">context</span> <span class="o">=</span> <span class="n">ProcessContext</span><span class="p">(</span><span class="n">processes</span><span class="p">,</span> <span class="n">error_queues</span><span class="p">)</span>
    <span class="k">if</span> <span class="ow">not</span> <span class="n">join</span><span class="p">:</span>
        <span class="k">return</span> <span class="n">context</span>

    <span class="c1"># Loop on join until it returns True or raises an exception.</span>
    <span class="k">while</span> <span class="ow">not</span> <span class="n">context</span><span class="o">.</span><span class="n">join</span><span class="p">():</span>
        <span class="k">pass</span>


<div class="viewcode-block" id="spawn"><a class="viewcode-back" href="../../../multiprocessing.html#torch.multiprocessing.spawn">[docs]</a><span class="k">def</span> <span class="nf">spawn</span><span class="p">(</span><span class="n">fn</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(),</span> <span class="n">nprocs</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">join</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">daemon</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="n">start_method</span><span class="o">=</span><span class="s1">&#39;spawn&#39;</span><span class="p">):</span>
    <span class="sa">r</span><span class="sd">&quot;&quot;&quot;Spawns ``nprocs`` processes that run ``fn`` with ``args``.</span>

<span class="sd">    If one of the processes exits with a non-zero exit status, the</span>
<span class="sd">    remaining processes are killed and an exception is raised with the</span>
<span class="sd">    cause of termination. In the case an exception was caught in the</span>
<span class="sd">    child process, it is forwarded and its traceback is included in</span>
<span class="sd">    the exception raised in the parent process.</span>

<span class="sd">    Arguments:</span>
<span class="sd">        fn (function): Function is called as the entrypoint of the</span>
<span class="sd">            spawned process. This function must be defined at the top</span>
<span class="sd">            level of a module so it can be pickled and spawned. This</span>
<span class="sd">            is a requirement imposed by multiprocessing.</span>

<span class="sd">            The function is called as ``fn(i, *args)``, where ``i`` is</span>
<span class="sd">            the process index and ``args`` is the passed through tuple</span>
<span class="sd">            of arguments.</span>

<span class="sd">        args (tuple): Arguments passed to ``fn``.</span>
<span class="sd">        nprocs (int): Number of processes to spawn.</span>
<span class="sd">        join (bool): Perform a blocking join on all processes.</span>
<span class="sd">        daemon (bool): The spawned processes&#39; daemon flag. If set to True,</span>
<span class="sd">                       daemonic processes will be created.</span>
<span class="sd">        start_method (string): (deprecated) this method will always use ``spawn``</span>
<span class="sd">                               as the start method. To use a different start method</span>
<span class="sd">                               use ``start_processes()``.</span>

<span class="sd">    Returns:</span>
<span class="sd">        None if ``join`` is ``True``,</span>
<span class="sd">        :class:`~ProcessContext` if ``join`` is ``False``</span>

<span class="sd">    &quot;&quot;&quot;</span>
    <span class="k">if</span> <span class="n">start_method</span> <span class="o">!=</span> <span class="s1">&#39;spawn&#39;</span><span class="p">:</span>
        <span class="n">msg</span> <span class="o">=</span> <span class="p">(</span><span class="s1">&#39;This method only supports start_method=spawn (got: </span><span class="si">%s</span><span class="s1">).</span><span class="se">\n</span><span class="s1">&#39;</span>
               <span class="s1">&#39;To use a different start_method use:</span><span class="se">\n\t\t</span><span class="s1">&#39;</span>
               <span class="s1">&#39; torch.multiprocessing.start_process(...)&#39;</span> <span class="o">%</span> <span class="n">start_method</span><span class="p">)</span>
        <span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="n">msg</span><span class="p">)</span>
    <span class="k">return</span> <span class="n">start_processes</span><span class="p">(</span><span class="n">fn</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">nprocs</span><span class="p">,</span> <span class="n">join</span><span class="p">,</span> <span class="n">daemon</span><span class="p">,</span> <span class="n">start_method</span><span class="o">=</span><span class="s1">&#39;spawn&#39;</span><span class="p">)</span></div>
</pre></div>

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